1. Field of the Invention
The present invention relates to a camera of which the focal length can be varied, and more particularly relates to a camera equipped with a zoom lens in which it is possible adequately to compensate for focus deviation during zooming caused by the increase in magnification ratio and the shortening of the closest object-to-camera distance.
2. Related Background Art
A camera with automatic focus adjustment device (hereinafter termed an autofocus camera) is per se known (for example refer to U.S. Pat. No. 3,442,193), in which there is incorporated a non-TTL (Through The Taking Lens) distance measurement device employing the principle of distance measurement by triangulation, i.e. of an active autofocus device or a passive type or the like. Further, various zoom lenses suitable for compact cameras with the lens shutters have been proposed, as for example the one described in U.S. Pat. No. 4,929,069, which it is hereby not intended to admit as prior art to the present application except to the extent in any case stipulated by applicable law. Such non-TTL distance measuring devices as described above are currently being applied to autofocus cameras in commercial production.
With this type of zoom lens, in order to provide for reduction in size of the camera from the point of view of portability, the distance (the overall length of the zoom lens) from the end of the zoom lens in the direction towards the object to be photographed (hereinafter referred to as "object") to the image plane (the plane of the film) is increased during zooming. FIG. 7 is a figure showing the situation before and after zooming such a zoom lens.
In a zoom lens, the focal length is changed by moving lens groups along the optical axis in a predetermined manner. In the present specification a set of mutually adjacent lenses which move together as a unit is termed a lens group, and this term includes such a set all of which are stationary. The term Z.sub.1 denotes a first lens group of the zoom lens, while the term Z.sub.2 denotes a second lens group of the zoom lens. The first lens group Z.sub.1, being the one by movement of which focus adjustment is performed, will therefore also be termed a focusing lens group; and it is moved forth or moved back (hereinafter referred to as "is projected") relative to the second lens group Z.sub.2 according to the photographic distance (i.e. object-to-camera distance) R from the object H to the image plane F, and thereby automatic focus adjustment is performed, so as to focus a sharp and clear image of the object onto the image plane.
In practice, the photographic distance R is obtained by calculation from (a) the distance to the object H as measured by a distance measurement device 1, i.e. the distance from said distance measurement device 1 to said object, and (b) the distance from said distance measurement device 1 to the image plane F, which is a distance characteristic of the camera.
As shown in FIG. 7, even in the case that the photographic distance R is not altered during zooming of the zoom lens (i.e. is the same before and after zooming), nevertheless, since the overall length of the zoom lens changes from l.sub.1 to l.sub.2 due to its being zoomed, therefore the distance from the end point Z.sub.f of the zoom lens to the object H changes from d.sub.1 to d.sub.2 when the lens is zoomed. Accordingly, even if it be provisionally assumed that the proper projection amount to the best focus position for the first lens group Z.sub.1 is not different before and after zooming, nevertheless, because the distance from the end point Z.sub.f of the zoom lens to the object H changes from d.sub.1 to d.sub.2 when the lens is zoomed, therefore after zooming focus deviation is bound to have been caused, even if focusing has been accurately performed before zooming.
In the prior art it has been possible to accommodate the amount of focus deviation associated with zooming within the focal depth of the zoom lens, and even without performing any particular focus correction it has in practice been possible to obtain properly focused photographs, even though the focus deviation has been neglected.
However zoom lenses of higher and higher magnification ratio, and of shorter and shorter closest photographic distance, have been contemplated recently, and the problem has arisen in their design and production that the amount of the above described focus deviation exceeds the focal depth of the zoom lens, so that the focus deviation can be neglected no longer.
In order to solve this problem, it has been conceived of to use a zoom lens whose overall length does not vary during zooming, as for example is disclosed in U.S. Pat. No. 4,497,547. However, this zoom lens presents lengthening and shortening difficulties from the point of view of portability, and is not well adapted for utilization with a compact camera.
Further, another means which has been contemplated for solution of the above outlined problem has been, during zooming, to move the non-TTL distance measurement device, which in the prior art was fixed with respect to the image plane, as one unit together with the focusing lens group of the zoom lens which is positioned closest to the object.
FIG. 8 is a figure showing a camera in which a focusing lens group provided at the end of the zoom lens closest to the object is moved as one unit together with a non-TTL distance measurement device. According to the operation of this FIG. 8 camera, the distances d.sub.3 and d.sub.4 before and after zooming from the end Z.sub.f of the zoom lens to the object H are equal to the distance value d.sub.0 as measured by the non-TTL distance measurement device 1A, and the result of this is that the above described type of focus deviation does not occur. However, with this method, it is necessary to move the non-TTL distance measurement device 1A as one unit together with the focusing lens group provided at the end Z.sub.f of the zoom lens closest to the object, and the mechanical arrangements required in order thus to move the distance measurement device 1A invite an increase in the size and the weight of the camera, and therefore this solution is not well adapted for application to the case of a compact camera, in the same fashion as was the case with the proposal of the aforementioned U.S. Pat. No. 4,497,547.